Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the pyrolysis furnace for further downstream processing, including quenching.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking low cost heavy feedstock such as, by way of non-limiting examples, crude oil and atmospheric resid, also known as atmospheric pipestill bottoms. Crude oil and atmospheric resid contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile, heavy ends of these feedstocks would lay down as coke in the convection section of conventional pyrolysis furnaces as the lighter components were vaporized. Only very low levels of non-volatiles can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude contaminated gas oils or naphthas which are contaminated with non-volatile components.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230 and 590° C. (450 and 1100° F.). The vapors are cracked in the pyrolysis furnace into olefins, and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
U.S. application Ser. No. 10/188,461 filed Jul. 3, 2002, Patent Application Publication US 2004/0004022 A1, published Jan. 8, 2004, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash vessel by maintaining a relatively constant temperature of the stream entering the flash vessel. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash vessel. The fluid can be water.
Provisional U.S. Patent Application Ser. No. 60/555,282, filed Mar. 22, 2004, which is incorporated herein by reference, describes a process for cracking heavy hydrocarbon feedstock which mixes heavy hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins, and the product effluent cooled in a transfer line exchanger, wherein the amount of fluid mixed with the feedstock is varied in accordance with a selected operating parameter of the process, e.g., temperature of the mixture stream before the mixture stream is flashed.
U.S. application Ser. No. 10/851,434, filed May 21, 2004, which is incorporated herein by reference, and U.S. Provisional Application Ser. No. 60/573,474, filed May 21, 2004, which is incorporated herein by reference, describe a process to increase the non-volatile removal efficiency in a flash drum used in a steam cracking system, the flash drum having a lower boot comprising an inlet for introducing stripping steam, a ring distributor for recycle quench oil, anti-swirl baffles, and a grate.
The present inventors have recognized that in using a flash/separation vessel to separate heavy non-volatile hydrocarbons from the lighter volatile hydrocarbons which can be cracked in the pyrolysis furnace, it is important to maximize the non-volatile hydrocarbon removal efficiency. Otherwise, heavy, coke-forming non-volatile hydrocarbons could be entrained in the vapor phase and carried overhead into the furnace creating coking problems in the convection section.
In simple flash operations, moderate visbreaking of hydrocarbon liquid will occur upstream of the drum and poor gas/liquid mass transfer exists in the piping upstream of the drum, resulting in production of light hydrocarbons which enter the bottom of the drum. Such drum bottoms can contain significant amounts of light hydrocarbons, which are lost as feed to the radiant section of the steam cracking furnace. Thus a significant opportunity exists to improve process economics by reducing the amount of light hydrocarbons present in the bottoms of the flash drum.
The present invention provides an apparatus and process for the effective removal of non-volatile hydrocarbon liquid from the volatile hydrocarbon vapor in the flash drum that treats the heated feed from the convection section of a steam cracking furnace. The present invention significantly enhances the separation of non-volatile and volatile hydrocarbons in the flash drum by modifying the lower boot of the flash drum to promote contact between a stripping gas and liquid entering the boot, e.g., from the upper flash drum or as recycled quenched liquid, thereby providing additional vaporization of light hydrocarbons from a given amount of feedstock.